This invention relates to control of the fluid drain and thence the temperature of steam-heated rollers. Especially but not exclusively in paper mills, a wet pulp is laid on a web and then is run over the surface of successive heated rollers to remove the water and to produce paper as a product. The flow of the pulp which is to become paper is very swift, and its path length through the machine is very long. Path velocities on the order of 35 miles per hour, widths of 24 feet or more, and roller diameters of five feet or larger are commonplace. The rollers are hollow drums to which steam is supplied, and from which condensate and steam are removed. In accordance with good thermodynamic practice, steam discharged from one roller is supplied to the next roller, often in a sequence of perhaps five rollers in a group.
The system is very massive. The mass transport is very large. In the event of failure such as breakage of the treated material or the failure of a roller or a group of rollers to perform their intended function, wastage of many tons occurs before the system can be shut down.
Paper mills are huge installations, and their capital investment is very great. A new paper mill can be expected to cost on the order of $800 million, and an older used mill, perhaps $350 million. Even these large costs do not reflect the total situation. The heavy wear to which they are subjected gives rise to a rebuilding cycle on the order of only about five years. Each five years the average paper machine will be shut in for a substantial period of time during which it is refurbished to operational standards, and at the same time is modernized to provide such advantages as may have become available since its last rebuilding.
The burden of the foregoing is that the costs of inefficiencies in these plants are literally immense. At the present time, in many paper mills that are regarded as in suitable condition, losses by way of unacceptable products approach 5% of the daily throughput. In a 1,000 ton per day mill, this amounts to 50 tons. This tonnage has consumed energy needed to drive the mill and to heat the product. Even though the scrap can be recycled, the labor and thermal energy are not recoverable. As a consequence, designers and operators of these mills are constantly alert for opportunities to reduce the generation of scrap, and to reduce the energy requirements. In fact, there are designers and rebuilders whose entire careers are occupied by redesign, improvement and refurbishment of paper mills with these objectives in mind.
What is genuinely surprising is the fact that in even the most advanced mill presently known to the inventor, control over the running conditions of the steam-heated rolls can still be improved so as to reduce the quantity of scrap produced by the mill. For example, with presently known controls, there is a substantial and continuing risk that the dryer roll may be flooded with hot water. The water greatly inhibits the heat transfer to the paper product. The present state of the art uses pressure drop across the dryer roll drain mechanism as the control reference. This is inadequate because some dryer rolls may be flooded, while adjacent dryer rolls are blowing steam through uncontrolled. Measurement of the supply steam and also of the drain steam as proposed by the instant invention provides BTU measurements needed properly to treat the paper.
At present the detection of a flooded dryer roll (which will not provide the proper drying conditions) is actually a happenstance thing. Periodically an inspector uses an infrared detector which he aims at the dryer roller to measure its temperature. He stands at a considerable distance from the roller and aims at a given spot on the rapidly rotating dryer roller. Assuming good aim and optimum conditions, a dryer roller at the wrong temperature will indeed be detected. But by the time a drop in temperature is detected, the dryer roller will be flooded with water. This causes off-specification paper to be produced that usually must be recycled with a loss of labor and energy.
However, even these proposed and imperfect results are rarely attained in mills of this type. This is because these mills are hot, humid, noisy, and very distracting places. As a consequence, a roller which is performing improperly is seldom discovered until the paper is not properly dried (scrap) or an associated large drive motor starts to overload. Meanwhile, as a precaution the entire system is often adjusted to dry the paper excessively so as to be certain the product is dried acceptably. This consumes excess energy, and can also lead to reduced production of paper.
The costs of these adverse circumstances are substantial. In a large mill, the costs of excess energy consumed but not truly needed can readily approximate $10,000 per day. The accurate control of roller conditions is, as can be seen, an objective long recognized by any prudent paper mill designer or operator.
Mills which utilize so many and such large rollers inherently must consume great amounts of energy, and for the intended applications the optimum source is steam. The properties of steam are well established, and it would seem to be a simple problem in thermodynamics to maintain the rollers in their correct condition both individually and as one of a sequence of rollers. The inefficiency of existing plants shows instead that it is not a simple matter after all, because after generations of good engineers have done their best (and produced large amounts of paper products), the problems and wastage continue to exist.
Surprisingly, the regulation of roller temperatures under the prevailing conditions by means of fluid flow measurements has not been done before, and by means of this invention, which utilizes this approach, the machines can reliably be regulated to assure that the individual rollers are indeed maintained at a proper temperature, with a proper BTU throughput.
It is an object of this invention to provide a rugged and reliable measurement and control which frustrates the flooding of a roller, or the converse, excess steam blow through, and which maintains throughput and temperature conditions to maintain a predetermined temperature while supplying the correct caloric output necessary to heat the paper while it resides on the rollers.
Importantly, this invention enables these results to be attained with the use of elegantly simple controls and minimized plumbing.